i dont exercise much at all. so im not much support for a marathon! i have sinus tachycardia and during pregnancy my resting heart rate is 120+, so working out, or even cleaning house, it gets to 140+. with the doppler my heart was usually faster than the babys! it is just too hard on me physically to really do much. i do stretch and walk a lot. i think the biggest worry would be dehydration, and maybe the lactic acid your body produces in intense exercise? good luck!